The present invention relates to the structure of an active region that is particularly suited for semiconductor power devices having a high breakdown voltage.
In recent years there has been intense development of new semiconductor materials (including so-called semi-insulating materials) for achieving semiconductor devices having special features, such as high-frequency characteristics, light emission characteristics, and withstand voltage characteristics. Among semiconductor materials, those with so-called semi-insulating properties in an intrinsic state, for example silicon carbide (SiC), gallium nitride (GaN), and gallium arsenide (GaAs), have higher hardness and are less susceptible to chemicals than silicon (Si), which is the most typical of semiconductor materials, and because these semiconductors have a large band gap, they have promise for future applications in next-generation power devices, high-frequency devices, and devices operating at high temperature, for example, utilizing their high withstand voltages.
Semiconductor power devices utilizing these wide band gap semiconductor materials include high withstand voltage Schottky diodes, MESFETs (Metal Semiconductor Field Effect Transistors), and MISFETs (Metal Insulator Semiconductor Field Effect Transistors), for example.
An example of a Schottky diode and a MISFET are provided here as conventional examples of a semiconductor power device.
FIG. 11 is a cross-sectional view showing the schematic structure of a conventional Schottky diode using silicon carbide (SiC). As shown in FIG. 11, numeral 101 denotes an n+ SiC substrate of approximately 100 xcexcm thickness that has been doped with a high concentration of nitrogen (N), which is an n-type carrier, numeral 102 denotes an nxe2x88x92 SiC layer that is approximately 10 xcexcm thick and has been doped to a low concentration of nitrogen (N), which is an n-type carrier, numeral 103 denotes a Schottky electrode made of a Ni alloy, numeral 104 denotes an ohmic electrode made of a Ni alloy, and numeral 105 denotes a guard ring made of SiO2. In this diode, when voltage is applied between the Schottky electrode 103 and the ohmic electrode 104 so that the Schottky electrode 103 has a higher potential than the ohmic electrode 104 (forward voltage), current flows between the Schottky electrode 103 and the ohmic electrode 104, and when voltage is applied between the Schottky electrode 103 and the ohmic electrode 104 so that the ohmic electrode 104 has a higher potential than the Schottky electrode 103 (reverse voltage), current does not flow between the Schottky electrode 103 and the ohmic electrode 104. That is, this Schottky diode has a rectification characteristic that allows current to flow in accordance with forward voltage, but blocks current with respect to reverse voltage.
However, there were the following problems with the above-described conventional Schottky diode.
Withstand voltage properties with respect to reverse voltage in the above-described conventional Schottky diode are highly dependant on the doping concentration in the nxe2x88x92 SiC layer 102. For example, to improve the withstand voltage of the Schottky diode, the doping concentration of the nxe2x88x92 SiC layer 102 in contact with the Schottky electrode 103 must be kept at a low level. However, because the resistivity of the nxe2x88x92 SiC layer 102 rises when the doping concentration is lowered, the on-resistance when forward voltage is applied becomes higher. The result is that power consumption increases. Because of this trade-off, it was difficult to simultaneously achieve a high withstand voltage and a low resistivity.
It is known that these problems occur not only in Schottky diodes but in MESFETs and MISFETs as well.
An object of the present invention is to achieve an active element having a high withstand voltage and low on resistance by creating a new structure for solving trade-offs such as those in the above-described conventional power devices.
The semiconductor device of the present invention is made by providing, on a substrate, an active region that functions as a portion of an active element, wherein the active region is configured by layering at least one first semiconductor layer which is provided on the substrate, and which functions as a carrier transit region; and at least one second semiconductor layer which includes a higher concentration of impurities for carriers than the first semiconductor layer, which has a thinner film thickness than the first semiconductor layer, and from which carriers can migrate to the first semiconductor layer due to quantum effects.
With this structure, quantum states occurs in the second semiconductor layer due to quantum effects, and the wave function of carriers that are localized in the second semiconductor layer comes to have a certain degree of widening. This results in the diffusion of carriers, such that carriers are not only present in the second semiconductor layer but also as the first conductor layer. Then, when the potential of the active region is increased and the carriers move, the carriers are continually supplied to both the second and the first semiconductor layers, and therefore the carriers are distributed such that they are always present in not only the second semiconductor layer but in the first semiconductor layer as well. In this state the carriers move not only through the second, but also through the first semiconductor layer, and thus the resistance value of the active region is reduced. In particular, because the scattering on impurity ions becomes smaller in the first semiconductor layer, a particularly high carrier mobility can be attained.
On the other hand, in a state wherein the entire active region has become depleted, carriers are no longer present in the active region, and therefore the withstand voltage properties depend on the first semiconductor layer, which has a low concentration of impurities, and over the entire active region a high withstand voltage value is obtained. This means that it becomes possible to simultaneously achieve low resistance and high withstand voltage of active elements, such as diodes and transistors, within semiconductor devices.
It is preferable that the first and second semiconductor layers are each provided in plurality and are layered in alternation. Thus, it is possible to more reliably achieve a low resistance value and high withstand voltage properties.
It is preferable that the concentration of impurities for carriers in the first semiconductor layer is below 1xc3x971017 atomsxc2x7cmxe2x88x923, and that the concentration of impurities for carriers in the second semiconductor layer is at least 1017 atomsxc2x7cmxe2x88x923.
It is preferable that the substrate and the active region are made of one material selected from SiC, GaN, and GaAs. Thus, it is possible to achieve a semiconductor device which has a structure suited for a power device in which materials having a wide band gap are used.
It is preferable that the first and second semiconductor layers in the active region are made of the same material. Thus, the potential barrier between the first semiconductor layer and the second semiconductor layer has an even smoother slope, and therefore it becomes easy for the carriers to be distributed across the first and second semiconductor layers in the active region.
It is preferable that if the second semiconductor layer is a SiC layer, the thickness of the second semiconductor layer is at least one monolayer and below 20 nm. Thus, in an operating state it becomes possible to effectively attain the migration of carriers into the first semiconductor layer.
It is preferable that if the first semiconductor layer is a SiC layer, then the thickness of the first semiconductor layer is at least about 10 nm and at most 100 nm. Thus, in an operating state, a certain amount of current can be secured.
It is preferable that the substrate is a semiconductor layer that includes a high concentration of impurities, that the uppermost portion of the active region is made of the first semiconductor layer, and that the semiconductor device further comprises a Schottky electrode providing a Schottky contact with a portion of the upper surface of the first semiconductor layer at the upper most portion of the active region, and an ohmic electrode providing an ohmic contact with a portion of the substrate. Thus, by taking advantage of the aforementioned characteristics of the active region, it is possible to obtain a vertical-type Schottky diode which is capable of simultaneously attaining a low resistivity during operation and a high withstand voltage value with respect to a reverse bias.
It is preferable that the semiconductor device further includes a Schottky electrode providing a Schottky contact with a first lateral face of the first semiconductor layer and of the second semiconductor layer of the active region, and an electrode that is connected to a second lateral face of the first semiconductor layer and of the second semiconductor layer of the active region, the second lateral face being arranged at a certain spacing from the first lateral face. Thus, by taking advantage of the aforementioned characteristics of the active region, it is possible to obtain a horizontal-type Schottky diode which is capable of simultaneously attaining a low resistivity during operation and a high withstand voltage value with respect to a reverse bias.
In the above case, it is preferable that the semiconductor device has a structure that further includes a doped layer for connecting lead, which is formed by introducing a high concentration of impurities into a region of the active region that is at a certain spacing from the first lateral face of the first semiconductor layer and the second semiconductor layer, and wherein the electrode is in ohmic contact with the doped layer for connecting lead.
It is preferable that the uppermost portion of the active region is made of the first semiconductor layer, and that the semiconductor device further comprises a Schottky gate electrode, which is in Schottky contact with a portion of the upper surface of the first semiconductor layer at the uppermost portion of the active region, and source and drain electrodes, which are provided on the active region and sandwich the Schottky gate electrode, and which are connected to the active region. Thus, taking advantage of the aforementioned characteristics of the active region, it is possible to attain a MESFET that is capable of achieving low power consumption, a high withstand voltage, and a high gain.
In that case, it is preferable that the semiconductor device has a structure which further includes two third semiconductor layers, which are provided on the active region and sandwich the Schottky gate electrode, and which include a high concentration of impurities,. and that the source and drain electrodes are in ohmic contact with the third semiconductor layers. Thus, it is possible to achieve a MESFET that has a recessed gate. structure.